java.lang.NoClassDefFoundError: com.google.ads.AdView
I had a similar problem, but did not have to add the libs folder; I tried, with no success, then removed them afterwards, when I realized that I had just forgotten to export the included Jar archived.
Fix:
Select project settings > Java Build Path, then Order and export.Here, check the GoogleAdMobAdsSdk-package.
This should solve the NoClassDefFoundError-issue.
Just create manually the "libs" folder, and add your libraries there. By default, they doesn't exist